Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
Packaging films and sheets are increasingly manufactured by coextrusion systems. These systems, in turn, are constantly growing in sophistication. Therefore, it is advantageous to develop polymers that can be manufactured by a variety of coextrusion systems, including blown film, cast film, injection molding, sheet/bottle, and others. Whereas some years ago it was common to have machines capable of coextruding three to five layer structures, today it is routine for machines to coextrude nine or more layers, for example by using an equal number of extruders to feed these layers.
Two common methods for producing these complicated multilayer structures are co-injection molding and injection stretch blow molding. Examples of these methods are described in detail in U.S. Pat. No. 6,974,556 and in European Patent No. EP2305751A1, respectively. These and other new coextrusion systems allow many types of polymers to be used together. Advantageously, these multilayer structures allow for better tailoring of the many properties that may be required in packaging structures, such as low permeability to water, gas or flavoring; toughness; sealability; gloss; transparency; or impact resistance. These tailored structures may also have a lower overall raw material cost, for example if they incorporate smaller amounts of the more expensive components.
Many packaging films and laminates contain layers of polar polymers such as polyamide or a copolymer of ethylene and vinyl alcohol (EVOH) to provide a barrier to the transport of gas and flavor. With the increasing number of extruders in coextrusion blown and cast systems, it is also common for a packaging film or sheet to include some combination of one or more polyamide layer and one or more EVOH layers. Tri-layer combinations used as barrier cores or “barrier sandwiches” comprise a coextrusion of polyamide/EVOH/polyamide layers and are widely employed in packaging films and sheets to improve barrier properties and formability.
In many cases, the exterior side of the coextrusion construction is composed of propylene polymers. The “exterior side” includes the outer layer(s), more particularly the layer(s) that are on the opposite side of the barrier film from the “interior side”. The interior side includes one or more inner layers, and in the case of a packaging material the innermost layer contacts the packaging contents. For flexible films, the propylene polymers provide stiffness and moisture barrier properties. They also provide a higher temperature resistance that prevents the film laminate from being deformed when contacted against a sealing bar during heat sealing of such laminates. The temperatures of the seal bars can be very high, particularly for thicker laminates, as heat must be transferred from the seal bar to the internal sealant layer within the short contact times necessary for profitable commercial production. Rigid structures, including coextruded thermoformed sheet, bottles and co-injection molded parts, frequently employ propylene homopolymers and copolymers as the bulking layers for rigidity, physical strength, moisture barrier properties and high temperature resistance that allows the contents of the package to be cooked and retorted.
Polar barrier layers such as those including EVOH are often employed in conjunction with the polypropylene layers so that the resulting packages are commercially viable as shelf stable products with no need for refrigeration. These various packaging structures with polypropylene layers and barrier layers require adhesive layers to bond the non-polar polypropylene layers to the polar barrier layers. The adhesive layers can vary in complexity of formulation to provide functional performance for different bonding requirements in a variety of packaging materials. For example, the packaging materials may have structures containing three layers, five layers, seven layers, up to thirteen layers, or more than thirteen layers. Adhesive concentrates, which can be diluted with polyolefins during the coextrusion process, provide flexibility to the converter in adjusting to different requirements, allow the converter to use the structural layers as adhesive layers as well where needed, and can be more economical than adhesive compositions that are pre-formulated with the polyolefin diluants.
Polypropylene-based adhesive concentrates in which the polypropylene is functionalized with high levels of grafted maleic anhydride typically have lower molecular weights than polyethylene-based adhesive concentrates of equivalent maleation because of the tendency of propylene polymer towards chain scission, also commonly referred to as “beta scission” or “vis breaking,” in the presence of peroxides. Without wishing to be held to theory, it is believed that functionalized polypropylene molecules having sufficiently low molecular weight must migrate from the bulk of the adhesive or “tie” layer to the interface with the polar polymer. There, the functionalized low molecular weight polypropylene molecules are available to react chemically or to form other adhesive interactions with the polar polymer. Disadvantageously, however, low molecular weight grafted polypropylene adhesives have been described as difficult to process, difficult to pelletize, and as having lower adhesion strength.
In coextrusion processes, the dissimilar layers that need to be bonded are in contact with the molten adhesive for a very short time, particularly in commercial production where the line speeds for some processes are very high. Therefore, useful polypropylene adhesives provide good adhesive strength in high speed coextrusion processes and have a suitable amount of functionalized polypropylene molecules that are sufficiently low in molecular weight to migrate to the interface with the polar polymer during the short contact time, while avoiding one or more disadvantages associated with lower molecular weight, for example, deficiencies in physical properties, such as rigidity and moisture transfer rates, and difficulties with pelletization.
Several approaches to increase the effective molecular weight of maleate-grafted polypropylene have been described. See, for example, U.S. Pat. No. 6,716,928, which describes functionalized propylene polymer products having higher amounts of grafted acid or acid derivative, preferably while maintaining relatively low MFRs. These grafted polymers may also be formulated into blends. See also U.S. Pat. No. 7,071,259, which describes the use of compositions comprising a mixture of two different functionalized propylene polymers as coupling/compatibilizing agents. Finally, U.S. Pat. No. 5,451,639 describes propylene copolymers grafted with α,β-ethylenically unsaturated carboxylic acids or carboxylic acid derivatives, methods of synthesizing the grafted propylene copolymers, and the use of the grafted propylene copolymers as adhesion promoters.
It is nevertheless apparent from the foregoing that there remains a need for an adhesive composition, and in particular for an adhesive concentrate, that provides high adhesion, good structural strength and other desired mechanical properties at low cost in multilayer coextruded structures that have from two or three layers to more than thirteen layers. The multilayer coextruded structures are particularly useful in packaging applications and have other useful and beneficial applications as well. The multilayer coextruded structures are especially useful in food packaging materials.